1. Field of the Invention
The invention relates to a drive apparatus for a vehicle, which includes an electric differential device that includes a differential mechanism that can perform a differential action and a motor; and a differential-action limitation device that limits the differential action of the differential device, and to a method of controlling a drive apparatus for a vehicle. More particularly, the invention relates to the limitation of a differential action using a differential-action limitation device.
2. Description of the Related Art
There is a known drive apparatus for a vehicle, which includes a differential mechanism that distributes an output from an engine to a first motor and an output shaft; a second motor provided between the output shaft of the differential mechanism, and a driving wheel; and a differential-action limitation device that limits the differential action of the differential mechanism. The mode of the differential mechanism is switched between a differential mode and a non-differential mode, based on an actual vehicle condition, according to a relation defined based on a vehicle condition. When the differential mechanism is in the differential mode, a differential action can be performed. When the differential mechanism is in the non-differential mode, the differential action is limited.
For example, Japanese Patent Application Publication No. 2005-206136 (JP-A-2005-206136) describes the above-described drive apparatus for a vehicle. In the drive apparatus for a vehicle, the differential mechanism includes, for example, a planetary gear unit. The main part of power output from the engine is mechanically transmitted to the driving wheel using the differential action, and the rest of the power output from the engine is electrically transmitted using an electric path from the first motor to the second motor. As a result, the differential mechanism functions as a transmission in which the speed ratio is continuously changed, for example, an electric continuously variable transmission. A control device controls the differential mechanism to drive a vehicle while maintaining the engine in an optimum operating state. Thus, fuel efficiency is improved.
In the differential mechanism, a clutch and a brake, which are regarded as the differential-action limitation device, are provided. By engaging the clutch or the brake, the mode of the differential mechanism is switched from the differential mode to the non-differential mode, that is, a locked mode. Thus, it is possible to obtain the drive apparatus that has the advantage of the transmission in which the speed ratio is electrically changed, that is, the effect of improving fuel efficiency, and the advantage of a gear type transmission device that mechanically transmits power, that is, high transmission efficiency. For example, by placing the differential mechanism in the differential mode when the vehicle is driven at a low-to-intermediate vehicle speed or at a low-to-intermediate output, that is, when the output from the engine is in a normal range, high fuel efficiency is achieved. By placing the differential mechanism in the locked mode when the vehicle is driven at a high vehicle speed and at a high output, the output from the engine is transmitted to the driving wheel only through the mechanical power transmission path. This prevents the loss in the conversion between the power and the electric energy, which would be caused when the differential mechanism is operated as the transmission in which the speed ratio is electrically changed. Thus, fuel efficiency is improved. In the case where the differential mechanism is placed in the locked mode when the vehicle is driven at a high vehicle speed, the differential mechanism functions as the transmission in which the speed ratio is electrically changed when the vehicle is driven at a low-to-intermediate vehicle speed and at a low-to-intermediate output. This reduces the maximum value of the electric energy that should be generated by the motor, that is, the maximum value of the electric energy that should be transmitted by the motor. This further reduces the size of the motor, or the drive apparatus for the vehicle, which includes the motor.
However, if a failure or the like occurs in an electromagnetic valve that supplies an engagement hydraulic pressure to engage the clutch or the brake, the differential mechanism is placed in the locked mode when the vehicle condition is in a region other than a region where the differential mechanism should be placed in the locked mode, that is, in a region where the differential mechanism should be placed in the differential mode, for example, in a region where the vehicle is driven at a low-to-intermediate vehicle speed and at a low-to-intermediate output. Therefore, the fuel efficiency of the vehicle may decrease, driveability may deteriorate, and a shock may occur.